device-tree bindings for rockchip 2D raster graphic acceleration controller (RGA)

RGA is a standalone 2D raster graphic acceleration unit. It accelerates 2D
graphics operations, such as point/line drawing, image scaling, rotation,
BitBLT, alpha blending and image blur/sharpness.

Required properties:
- compatible: value should be one of the following
  "rockchip,rk3228-rga", "rockchip,rk3288-rga": for Rockchip RK3228
  "rockchip,rk3288-rga": for Rockchip RK3288
  "rockchip,rk3399-rga": for Rockchip RK3399

- interrupts: RGA interrupt specifier.

- clocks: phandle to RGA sclk/hclk/aclk clocks

- clock-names: should be "aclk", "hclk" and "sclk"

- resets: Must contain an entry for each entry in reset-names.
  See ../reset/reset.txt for details.
- reset-names: should be "core", "axi" and "ahb"

Example:
SoC-specific DT entry:
	rga: rga@ff680000 {
		compatible = "rockchip,rk3399-rga";
		reg = <0xff680000 0x10000>;
		interrupts = <GIC_SPI 55 IRQ_TYPE_LEVEL_HIGH>;
		clocks = <&cru ACLK_RGA>, <&cru HCLK_RGA>, <&cru SCLK_RGA_CORE>;
		clock-names = "aclk", "hclk", "sclk";

		resets = <&cru SRST_RGA_CORE>, <&cru SRST_A_RGA>, <&cru SRST_H_RGA>;
		reset-names = "core, "axi", "ahb";
	};